Abstract

We have investigated orbital excitations in LaMnO3, RTiO3 (R = La,Sm,Y) and TiOX (X = Cl,Br) by optical spectroscopy. Peaks in Raman data of LaMnO3 have been interpreted in the literature as first experimental evidence of novel orbital excitations with a significant dispersion. In the optical conductivity we observe peaks at the same energies as the peaks in the Raman data. This is in contradiction to the fact that the observation of orbital excitations in the optical conductivity requires the excitation of an additional phonon in order to break the dipole selection rule. Thus orbital excitations should be shifted by the phonon energy which typically amounts to 50-80 meV. Therefore we attribute the peaks to two-phonon processes. Further we observe a shoulder at the onset of the first electronic excitation. By comparison with the results of a cluster calculation it is attributed to a local crystal-field excitation. From this we conclude that the coupling to the lattice is the dominant mechanism that lifts the degeneracy of the eg orbitals in LaMnO3. In RTiO3 (R = La,Sm,Y) the role of the orbital degree of freedom is discussed controversially in the literature. For LaTiO3 a novel orbital ground state of strong orbital fluctuations has been proposed. On the other hand a sizeable distortion of the cubic symmetry has been observed. This distortion suggests an orbitally ordered ground state. We have determined the optical conductivity of RTiO3 (R = La, Sm, Y) from reflectance and transmittance data. We found a broad peak at about 0.3 eV in all three compounds. The peak energies as well as the line shape are in good agreement with a crystal-field scenario. For such a large intra-t2g splitting a significant role of fluctuations can be ruled out. However, a back door has been opened up for the orbital-liquid picture by assuming that the large observed energy actually corresponds to a two-orbiton process. The fact that only one peak is observed and that its line shape is not very characteristic makes it impossible to draw a final conclusion for the ground state of LaTiO3. In YTiO3 orbital order has been observed experimentally. But the role of orbital fluctuations is still under discussion. This compound shows a significant polarization dependence. This observation is in agreement with the crystal-field scenario. However, in an orbital-liquid scenario the pure orbital excitation is predicted to be isotropic since in this scenario cubic symmetry persists. In the light of our results on YTiO3 the dominant role of orbital order in this compound becomes apparent. This definitely favors the description of YTiO3 within the crystal-field scenario. The bilayer system TiOX (X = Cl,Br) forms a quasi-1D spin system because of the orbital occupation. The magnetic susceptibility is well described in terms of a S = 1/2 Heisenberg chain at high temperatures. Below a temperature Tc1 the susceptibility vanishes which has been attributed in the literature to a spin-Peierls transition. An additional kink at Tc2 > Tc1 has been discussed in connection with orbital fluctuations. In the transmittance data we observe absorption features below the band gap. These features have been assigned to orbital excitations. The data are described very well by a cluster calculation, which yields a t2g splitting of 0.65 eV. This assignment is corroborated by ESR data which give a g-factor of approximately 2. For a splitting of 0.65 eV within the t2g orbitals orbital fluctuations can be neglected. We have shown that the bilayer geometry is responsible for the unconventional second phase transition. Frustrated interchain coupling leads to a second-order phase transition to an incommensurate spin-Peierls phase below Tc2. At Tc1 the fully dimerized spin-Peierls phase locks in by a first-order phase transition. Experimental evidence for this scenario is found in the phonon spectra where changes are observed at Tc1 and Tc2. This indicates that the lattice is involved in both transitions. Moreover, in the intermediate phase phonon modes are observed which are absent in the low- and the high-temperature phase. This indicates a lower symmetry, as expected for the incommensurate phase.